These days, in wireless communication, high-efficiency transmission through digitization is frequently employed. If a multi-level phase modulation (phase shift keying) method is used in wireless communication, it is important that leakage of power to adjacent channels be reduced by suppressing nonlinear distortion at a transmission side particularly by linearizing amplification characteristics of a transmission power amplifier.
Also, if power efficiency is to be improved by use of an amplifier having poor linearity, a technique for compensating for nonlinear distortion generated by such an amplifier is employed.
For example, the following distortion compensation processing called “predistortion” is known. Generally, “predistortion” is carried out by generating distortion in advance of a transmission signal being amplified, by multiplying the transmission signal by a distortion compensation coefficient so as to cancel out distortion generated in an amplifier. In this distortion compensation processing, a transmission signal amplified by an amplifier is fed back and is compared with an original transmission signal. Then, the distortion compensation coefficient is updated so that the difference between the transmission signals before and after amplification is minimized. Thus, the distortion compensation coefficient converges to the optimal value, and distortion of a transmission signal generated in the amplifier is compensated for.
The distortion compensation coefficient is calculated and updated in association with each of the discrete power levels of transmission signals.